Aircraft turbine engines are structured with a large fan which rotates at high speeds during engine operation. The maximum recommended speed of the fan is a function of several environmental conditions including the total air temperature, the pressure altitude, and the altitude rate. The maximum suggested engine fan speed, the overspeed limit value, is also a function of the operation of the aircraft; that is, the overspeed limit value is dependent upon whether the airplane is in takeoff, climb, or cruise operation. Electronic monitors to maintain the actual turbine fan speed below the overspeed limit value by controlling the flow of fuel to the engines are well known in the art. The typical fan speed monitor senses air temperature and pressure altitude and electronically selects an overspeed limit value from a manufacturer's list of recommended maximum engine fan speeds for each given temperature and pressure altitude. An overspeed condition existed whenever an actual engine fan speed exceeded a certain percentage of this recommended maximum engine fan speed. The typical fan speed monitor reduced the fuel flow to the engines whenever the fan speed exceeded the predetermined percentage.
For example, U.S. Pat. No. 4,467,599 to Moore discloses a fan speed control system that prevents an overspeed condition. The Moore apparatus automatically prevents the turbine fan from exceeding the overspeed limit by controlling the fuel flow to the engine. As shown in FIG. 1 of Moore, a pressure altitude sensor and a temperature probe determine altitude and temperature values which are then used to calculate the overspeed limit value. A limiter 40 insures that the turbine fan does not rotate faster than this overspeed limit by restricting the fuel flow to the jet engine whenever an overspeed condition exists.
However, the fan speed monitor according to Moore does not calculate the overspeed limit value as a function of either altitude rate or the position of the landing gear. Since the fan speed monitor according Moore calculates the overspeed limit value only as a function of total air temperature, pressure altitude and whether subordinate systems are operative, without reference to the change in altitude rate or the position of the wheels, the overspeed limit value determined by Moore is an imprecise value. Further, the Moore monitor cannot display the overspeed limit values or record the cumulative time during which these values have been exceeded during the flight. Moore thus does not indicate to the pilot when an overspeed condition has occurred. Moore also does not provide a means for maintenance personnel to find out, after the flight, the existence and duration of any overspeed condition. Such information regarding the existence and duration of any overspeed conditions is useful to determine the continued serviceability of the engine and the need for corrective maintenance. Finally, the apparatus to Moore does not allow the pilot or maintenance crews to input data from previous flights to progressively compile historical overspeed data for a particular engine.
U.S. Pat. No. 4,712,372 to Dickey provides an overspeed system redundancy monitor that determines a performance status of the overspeed system in an aircraft during the course of previous flights. However, this monitor controls the rotational speed of the turbine with respect to a single overspeed limit and does not determine the overspeed limit values using the measured conditions such as pressure altitude, total air temperature, change in altitude rate, and landing gear position.
Therefore, a fan speed monitor is needed which continuously calculates overspeed limits as a function of total air temperature, pressure altitude, altitude rate, and wheel position. Further, a fan speed monitor is needed which provides an instantaneous warning to the pilot and maintenance crews of an overspeed occurrence and makes a record of the frequency of overspeed occurrences as well as the time and duration of such occurrences. Finally, a fan speed monitor is needed which can be programmed with historical data such that as an engine is replaced, the previously accumulated historical data for that engine can be programmed into the monitor to continue the data accumulation, rather than begin compiling data anew.